The present disclosure relates to an image forming apparatus including a fixing device for executing a fixing process on a sheet on which a toner image has been formed.
Conventionally, a fixing device is provided in an image forming apparatus, such as a copier, a printer, and a facsimile which include an electrophotographic image forming portion, and a multifunction peripheral in which functions of these are installed. The fixing device includes a fixing roller and a pressure roller, wherein the surface of the fixing roller is heated by a heat source, and the pressure roller is configured to be rotated while being pressed against the fixing roller. When a sheet with a toner image formed on the surface thereof passes through the fixing device, the sheet is heated while being nipped by the pressure roller and the fixing roller with a certain pressure. This allows the toner to be fused and adhered to the sheet, thereby the toner image is fixed to the sheet.
In general, the heat source provided in the fixing device is controlled such that a surface temperature of the fixing roller reaches a certain target value, the surface temperature being detected by a temperature detecting element such as a thermistor. As the method for controlling the temperature in this way, a so-called ON/OFF control method may be adopted, wherein according to the ON/OFF control method, electricity supply to the heat source is turned ON when the detection temperature is lower than the target value, and turned OFF when the detection temperature is higher than the target value. In that case, however, the temperature of the fixing roller repeats an overshoot and an undershoot with a relatively large amount of overshoot and undershoot, respectively.
In view of the above, the PID (Proportional Integral Derivative Controller) control method may be used as another method for controlling the heat source. The PID control method is a control method for controlling the amount of electricity supply by using, as a control value, a sum of: a value proportional to a deviation between the current temperature and the target value; a value proportional to the integral of the deviation; and a value proportional to the differential of the deviation. In the case of the PID control method, the detected temperature of the fixing roller gradually approaches the target value, without large overshoot or undershoot as in the ON/OFF control method.
Meanwhile, from the viewpoint of improving the user convenience by reducing the warm-up time or the like, the image forming apparatus is required to be able to raise the temperature as quickly as possible when the image forming apparatus is activated, or when it returns from the energy saving mode to the normal mode. In this respect, the PID control method is not suitable as a temperature control method used during the activation of the image forming apparatus or the like because the PID control method allows the temperature of the fixing roller to change with a small change rate.
Thus it may be considered that the ON/OFF control method, which allows the temperature of the fixing roller to change with a relatively large change rate, is used to raise the temperature quickly during the activation of the image forming apparatus or the like, and switches the temperature control method to the PID control method as the temperature of the fixing roller approaches the target value. By performing such a temperature control, it is possible to restrict the overshoot and undershoot, while allowing the temperature of the fixing roller to be stabilized to the target value as quickly as possible.